The invention generally relates to materials and methods for creating and/or utilizing upconverting luminescence. More particularly, the invention relates to novel compositions (e.g., nanoparticles) and related methods of preparation and use that enable upconverting luminescence with an efficient excitation optimized at about 800 nm. A unique class of cascade sensitized tri-doped UCNPs with a biocompatiable 800 nm excitable property are disclosed herein, for example, tri-doped β-NaYF4:Nd, Yb, Er (Tm)/NaYF4UCNPs, which employ Nd3+ as 800 nm photon sensitizer and Yb3+ as bridging ions, having strong green or blue upconversion emissions without photobleaching.
